The plans call for a piece of 25 mm round brass bar as the starting point for the cylinder. I found one small piece among some offcuts on my shelf. It was only a little longer than the required 30 mm and by the time I had faced it at both ends (which entailed a silly mistake that I should not have made) it was 29.65 mm long. I know that 0.35 mm is actually quite a lot, but I assume that in a single acting oscillator the length of the cylinder isn't critical to that degree. The alternative would be to order some more brass bar online (as these materials are hard to get hold of where I live) and that would seem to be overkill for 0.35 mm off the length of an oscillator cylinder. However, please correct me if I'm wrong.

The main axle is 6.0 mm in diameter in the plans. The nearest I had was round bar of 6.25 mm diameter in what I think is silver steel (it came as part of a whole bunch of stock with a Myford lathe that I bought from a man who had built a loco). I turned a length of it down with very light cuts and polished it with emery paper. It's now sitting at about 6.10 mm and I think it will need some further polishing but I'll leave this until my new reamers arrive and make a 6 mm reamed hole to test the axle with so I can hopefully bring it to a nice running fit. The length is arbitrary at this point - I deliberately left it on the long side because I might put a pulley on it at the end of the build, and in any case better too long than too short at this point.

Pictures:

I'm unlikely to be doing anything to the engine now for about a week as I have to work for the next two days and am then visiting my daughter for a long weekend...

Hi Gary, it’s coming on well and will look interesting when you have it finished. (Unlike my square blocks.)

While 0.35 mm is a long way out for a 3 mm bolt hole, let alone the shaft bearing diameter, it may not matter at all, or even improve things when in the cylinder length.

Draw a little sketch with the crank at top dead centre. Calculate the distance through the frame from the crank shaft cylinder pivot pin and inside of the cylinder top head. Then calculate from the shaft through the crank pin at top dead center, piston rod and piston. Then you can do the subtraction to see how much clearance there is above the top of the piston. If it iis supposed to be around millimetre, a reduction by 0.35 will not matter. The main thing is to keep an eye on that clearance at top dead centre and not allow it to disappear as other components are made with their inevitable tolerances which inevitably add up the wrong way. That is what Murphy’s law is all about.

The piston sides are the bearings through which the piston causes the cylinder to oscillate, and longer is better, so long as the bottom of the piston stays within the cylinder. So if you do run out of clearance at TDC, I would suggest making the piston rod a little shorter rather than shorten the piston. Though you also have to check that the crank web/disk clears the bottom of the piston (and cylinder) as it goes through TDC.

That is why an oscillator is such a great place to start, even with such a simple engine, there is so much to learn as you build it up. With more parts in a more complex engine, there are more places where the dimensions of one part affect the clearances for another. Part of the preparation before you start on a more complex model, is to identify these critical dimensions.

Like your boiler project, you are learning so much from each step. Well done.

I take your point about shortening the rod rather than the piston. I didn't get it at first until I realised what you said, i.e. the piston causes the cylinder to oscillate (or at least is the final link in the system that does so), and the force for the lateral movement is transmitted through the sides of the piston. It seems to me that for this reason piston length could be more important in an oscillator than in a fixed cylinder engine...?

Not sure how much difference that 0.35 mm will make in my case, but I will carry out the calculation you suggest because I plan to have a go at machining the piston assembly as one, so it won't be possible to shorten just the rod after it's made. It would be different if the piston head and the rod were two components.

Hi Gary, yes you have got it on the piston length. The calculation will tell you if you need to make any adjustments to the position of the crank pin bearing location before you finish off that end of the rod.

And yes, if the design has a rod packing and cross head, both of which keep it pretty square, the piston does not have to be so long. Just enough for rings, or grooves or what ever you are doing to reduce leakage past the piston. But at the same time, those same components have to be lined up very well or will result in extra friction, which is an extra complication for a beginner. You already know more about what you will need to do, just from the experience of building that oscillating engine, things that are not always so obvious from a first look at a design. Drilling a hole is easy, getting it in the right place is more difficult, and if you really need a precise location or alignment, another level again.

Thanks for your comments about my engines. I think I was a bit harsh on myself, after all Elmer’s engines are mostly square, along with so many other designs, and they all look pretty good. I was just saying that I don’t have the artistic flare that you clearly demonstrate in your projects.

Yes - I'm with you regarding the piston. The calculation will tell me by how much the crank pin bearing needs to move by, as you say. That's the right way round to look at it. Even if the difference in this case proves to be insignificant in practice, it will be good experience for me to do the sums anyway.

Agree re Elmer's engines - there was a real aesthetic sensibility going on there, alongside the creative engineering. A lot of them are quite rectilinear in style but graceful nonetheless in a modernist kind of way. His #36 reversing wobbler is a good example of that I think.

Just a thought Gary but if you are interested in making engines with an artistic flair have you considered buying a set of French curves?

That was a good suggestion Peter, and in response I have ordered these:

I won't need the actual French curves until I start another project of this kind, but the small circle templates will come in very handy for laying out the decorative holes that I plan to make in the standard of this one when it's done.

You can likely adjust the thickness of the piston to compensate for the slightly short cylinder. Shouldn't be a big issue.

Bill

Bill - I'm sure that's right, but having thought a bit more, given I'm a newbie at this and working from the plans, I decided not to take any risks with getting the ports to line up as exactly as possible. Probably being over-fussy but no harm. I also remembered that to get more bar I wouldn't need to order online, as a local small engineering firm owned by one Adrian have a scrap bin which is a veritable treasure trove. I found a piece of 25 mm brass bar at 37 mm long, which he kindly gave me for nothing. Perfect!

I'll turn it down to 30 mm and carry on with the build next week when I come back from my daughter's.

The curves look to be knock offs of Rotring ones and some orange copies of Linex templates for the others.

Regarding the cylinder with such a small difference in length you could just leave the end cap a little thinner and have the piston travel closer to the open end, provided you set out the ports from the pivot point it won't make any difference and you can put the extra brass away for the next engine.

@ Jason: I have no doubt you are correct. However, I've already started on the new bit of brass (quick fix of shop for 20 minutes before I head off) and in any case it will be good practice for me to work to a plan as exactly as I can (apart from the frame and base of course!) for my first build. I guess a better understanding of the various tolerances will come in time. I'll keep the first bit of brass for the next engine, so nothing wasted.

Hello Gary, this is the first engine that I made, mainly because it had no silver soldering. I just measured my cylinder and it is 29.85mm the piston protrudes 1.84mm when at BDC. It runs really well and will tick over at 2psi. One thing that I did find was that I lapped the cylinder and piston and then drilled holes for the endcap bolt threads. These made a slight "bulge" into the cylinder so I had to lap again. Next time I will drill and tap holes before final reaming/lapping. As several people have said it will take days to get that silly grin off your face when it first runs. I am now on my second engine a very simple beam engine. Best of luck John

My daughter lives in Hove, East Sussex, on the South coast of England. We spent a very nice weekend hanging out together - we're like pals really. In Hove there is a Victorian pumping station which was converted to a steam museum some years ago - The British Engineerium. There are two engine houses, each containing a lovely original beam engine, I believe. Unfortunately it closed several years ago and has been standing intact but unused - and up for sale - ever since.

I was aware that it was closed but decided to make a pilgrimage there anyway, just to stand outside the inevitable locked gate:

What a waste. I wish someone would buy it and reopen it. Me - I can't afford it. How about you guys?

I did, however, decide that I could just about afford this nice old hand vice which I found in a huge second-hand emporium in the neighbouring city of Brighton (one of the coolest cities anywhere, never mind the UK). Ten quid. Not bad, I reckon: